In Europe, tractors are used for over-the-highway purposes more than they are in the U.S. When so used, the tractors are always in a two-wheel drive mode for best efficiency and reduced drive line loads. European standards require four-wheel assisted braking for relatively high speed tractors. Therefore, tractors having brakes on the rear wheels only are provided with a front wheel drive clutch that is engaged when a brake pedal is actuated so that braking of the rear wheels also causes braking of the front wheels. U.S. Pat. Nos. 5,248,020, 4,951, 775, 4,811,811 and 4,466,519 illustrate braking systems of this type.
In prior art braking systems the front wheel drive clutch is too aggressive so that during hard braking the front wheels lock up. This causes front wheel skidding and tractor bouncing which in turn may possibly damage the driveline components of the front wheel drive.